1. Field of the Invention
The present invention relates to a load drive apparatus to drive a load by a speed-controllable motor.
2. Description of the Prior Art
A recent demand is to establish a technique of smoothly driving a load at extremely low speed. The prior art to meet such demand is in general to use a transmission apparatus which is able to greatly reduce motor speed. However, the use of the transmission apparatus not only makes the entire load drive apparatus considerably large, but also lowers operating efficiency.
To find a solution for the above mentioned problem, various attempts have been made to operate the motor directly without using a transmission apparatus. One example is provided by a Japanese Pat. No. S58-9596 disclosed in the official gazette.
Now, in many cases for motor speed control, the operating speed of the motor is detected and used as a feedback signal. A means of detecting this motor speed is of two types, one to output analog signals and the other to output digital signals.
As a means outputting digital signals is less affected by temperature variation than that of outputting analog signals, the use of digital signal for this purpose is becoming the main stream in this field.
However, if a device that outputs a digital signal is used as a speed detecting means for the motor in operation at an extremly low speed of less than 1 [r.p.m], as an example, the interval of pulses output from this speed detecting device, becomes considerably large, thus being unable to know the actual motor speed during the pulse interval. To fill this blank, the preceding speed information obtained is held with the assumption that the motor is currently in operation at this speed to drive the motor.
Therefore, if the motor speed information obtained indicates greater than the actual speed of revolution, the motor speed is reduced less than a specified speed of revolution or the motor stops in the worst case, thus eventually failing in smooth motor operation.